1. Field of the Invention
The present invention relates generally to defibrillation processes, and more particularly, pertains to the new process of pretreatment, especially electrically rendering the heart muscle more amenable than it would otherwise be to the influence of defibrillation waveforms.
2. Description of the Prior Art
Defibrillation, or causing the cessation of chaotic and uncoordinated contraction of the ventricular myocardium by application of an electrical voltage and current, in its most primitive form goes back to the last century. [J. L. Prevost and F. Batelli, "Spur quelques effects des descharges electriques sur le couer des mammifers", Comptes rendus hebdomadaires des sanes de l'Academie des sciences, Vol. 129, pp 1267, 1899.] The sophistication and effectiveness of defibrillation techniques has grown rapidly in recent decades. One of the most recent developments has been the practical advent of implantable defibrillation systems. [R. A. Winkle, R. H. Mead, M. A. Ruder, et al., "Long-term outcome with the implantable cardioverter-defibrillator", J Am Coll Cardiol., vol 13, pp 1353-1361, May, 1989. M. H. Lehman, S. Saksena, "Implantable cardioverter-defibrillators in cardiovascular practice: Report of the policy conference of the North American Society of Pacing and Electrophysiology", PACE, vol 14, pp 969-979, June, 1991.] With the acceptance of this technology, the new challenge is to reduce system size while preserving its effectiveness, in order to improve the patient's quality of life and to extend the range of application of such systems. [R. A. Winkle, "State-of-the-Art of the AICD", PACE, vol 14, pp 961-966, May, 1991, pt II. N. G. Tullo, S. Saksena, R. B. Krol, "Technological improvements in future implantable defibrillators", CARDIO, vol 7, pp 107-111, May, 1990.]
The next challenge, not yet met, is to develop ways to detect imminent fibrillation so that sophisticated pacing techniques may forestall its onset, thus sparing the patient the discomfort and trauma of a high-energy electrical discharge within the upper body. [J. E. Skinner, C. Carpeggiani, C. E. Landisman, et al., "Correlation dimension of heartbeat interval is reduced in conscious pigs by myocardial ischemia", Cir. Res., vol 68, pp 966-976, April, 1991. M. W. Kroll and K. W. Fulton, "Slope filtered correlation dimension algorithm and its evaluation with pre-fibrillation heart rate data", J. Electrocardiology, vol 24, January, 1992.] Until an ability to anticipate fibrillation has been achieved, it will be necessary to achieve defibrillation by passing a large current through the heart using a system such as that represented schematically in FIG. 1, the prior art. The current must be large enough to extinguish wavefronts by directly depolarizing a major portion of the myocardium. [D. P. Zipes, J. Fischer, R. M. King, et al., "Termination of ventricular fibrillation in dogs by depolarizing a critical amount of myocardium", Am Cardiol., vol 36, pp 37-44, July, 1975.] The current value must also be above the defibrillation threshold, and above an upper limit of ventricular vulnerability so that cells will not be stimulated in a fashion that causes fibrillation to commence anew. [P. S. Chen, N. Shibata, E. G. Dixon, et al., "Comparison of the defibrillation threshold and the upper limit of ventricular vulnerability", Circulation, vol. 73 #5, pp 102-1028, May, 1986.]
Prior art in the broadest sense uses an arrangement like that shown schematically in FIG. 1 to deliver electrical stimulus to the heart. A typical waveform, voltage versus time, employed for defibrillation may be the monophasic waveform pictured in FIG. 2, the prior art. Typical values are a pulse duration of 7 milliseconds, a peak amplitude of 750 volts and peak current of 15 amperes, assuming electrodes that yield a heart resistance of approximately 50 ohms. A minor variation of this prior art case involved one pulse delivered between a certain pair of electrodes, and a second pulse delivered through a different pair of electrodes.
Another prior art option that shows improved results is the biphasic waveform depicted in FIG. 3, the prior art. [E. G. Dixon, A. S. L. Tang, P. D. Wolf, et al., "Improved defibrillation thresholds with large contoured epicardial electrodes and biphasic waveforms", Circulation, vol 76 #5, pp 1176-1184, Nov., 1987.] Here, the initial pulse is similar to the monophasic pulse, but a second pulse of opposite polarity is caused to follow the first immediately by a switching reversal of the capacitor.
The processes and procedures of the prior art that most nearly approach the novel feature of the present invention, however, are quite remote from it. Hence, their marginal relevance can best be appreciated by first noting the outline of the present invention.
In one embodiment of the present invention, the short-duration, high-voltage defibrillating pulse is preceded by a contiguous and same-polarity, long-duration, low-voltage waveform. In other embodiments of the present invention, a train of electrical pulses requiring little energy storage achieves temporal and spatial organization of significant portions of the heart muscle, so that a subsequent defibrillating pulse can accomplish its role using less energy than would otherwise be required. The pulse train is delivered through the large electrodes used to administer the defibrillating pulse.
The only even remotely similar methods in the prior art are these: One could argue that in the biphasic defibrillation of the prior art, the initial pulse constitutes an electrical pretreatment that improves the efficacy of the second, although several significant elements of the present invention are absent altogether in this case, particularly the train of pulses of the other embodiments. A further point of difference is that our pretreatment pulse or pulses are of significantly lower energy than the defibrillation pulse, whereas the first pulse in the biphasic waveform is usually of higher energy than the second.
Pulses delivered at a rapid rate are sometimes used in the prior art of pacing to combat tachycardia, or rapid pulse. The aim here is to "outrun" the tachycardia by administering heart-stimulating pulses at a high rate. This pulse-train case is very different from those of the present invention, however, because (1) it delivers pulses through the pacing lead rather than through the large defibrillation electrodes; (2) it employs pulse voltages in the neighborhood of 5 volts rather than several hundred volts; (3) it seeks to achieve a cessation of tachycardia rather than fibrillation; (4) its pulse train delivered through the pacing lead is never followed by a significantly stronger pulse, whereas in embodiments of the present invention, the train of pulses delivered through the defibrillation electrodes is always followed by a significantly stronger pulse.
It should be noted that the dual-pulse, single-path option was studied in the 1960s and found to yield no improvement. [Kugelberg, Jan. "Ventricular defibrillation: a new aspect", Acta Chirurgica Scandinavica, Supp 372, Stockholm, 1967.]
Another aspect of prior art is the dual-pulse process discussed above as a variation on the monophasic waveform, wherein the initial pulse could conceivably be termed "pretreatment". But this method differs fundamentally form the present invention because a different path is used for the second path in the dual-pulse method of the prior art. Other prior art is the work of Jones and Jones. [J. L. Jones and R. E. Jones, U.S. Pat. No. 4,637,397, filed May 30, 1985, and issued Jan. 20, 1987.] It employs what is essentially a biphasic pulse, followed by a comparatively low-power "healing pulse", the last having the same polarity as the first pulse. The concept of a final low-power or healing pulse is totally absent from the present invention.
Summarizing, the present invention differs from the prior art in that it uses (1) many pulses or else one or more long-duration, low-voltage pulse; (2) high-heart-rate pulse spacings greater than, for example 200 milliseconds, while the dual-pulse spacing is negligible, or the order of 1 millisecond; (3) the same pair of electrodes for all pulses; (4) one or many low-energy pulses followed by a high-energy pulse, rather than two or more pulses of comparable energy.